feat(workspace): route platform Back gestures to the header Back (#1943)

* feat(workspace): route platform Back gestures to the header Back

The mouse Back button, Alt+Left (Cmd+[ on macOS) and, in Electron, the
window's app-command browser-backward (Windows/Linux) and trackpad swipe
(macOS) now run the workspace header's Back, one step each.

- WorkspaceBackGesturesService (shell-provided) dispatches every gesture
  through WorkspaceBackNavigationService.goBack(). A gesture is swallowed
  while a dialog, menu, the phone drawer or a fullscreen player is up, and
  left to the browser only where the header has no Back.
- The chord is a navigate-back entry in the shortcuts dialog, handled in a
  capture-phase listener of WorkspaceKeyboardShortcutsService; ignored while
  typing, repeats swallowed.
- Electron forwards app-command and swipe over the push-only channel
  WINDOW:BACK_REQUESTED (preload onWindowBackRequested), gated in the
  renderer on RuntimeCapabilitiesService.supportsWindowBackGestures.
- A mouse press that also arrives as app-command goes back once, however
  long the button is held; Electron's own Chromium history step on the
  mouse button is always prevented.
- No Forward binding.

Docs: workspace-shell (Platform gestures), portal-detail-navigation,
electron-security, UI guidelines, README shortcuts table.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* refactor(workspace): drop NgZone from the Back gestures service

Master runs zoneless change detection and no longer uses NgZone; the
bridge callback calls goBack() directly.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fix(workspace): keep Back gestures where the Fullscreen API is missing

iPhone Safari has no unprefixed document.fullscreenElement; the
undefined value read as "a player is fullscreen" and blocked every
gesture.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fix(workspace): keep a held Back chord's repeats swallowed after Back

Repeats asked for the current result, so once the first press reached a
page with no Back left they were released to the browser, which could take
another history step. Repeats now follow the chord's first press until a
key is released.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fix(electron): one Back per press of the Linux Browser Back key

On Linux Electron raises app-command browser-backward for every
unhandled event of the Browser Back key (NativeWindowViews::
HandleKeyboardEvent), keydown and keyup alike, so one press went back
twice. The main process now consumes that key in before-input-event and
requests Back once per press; mouse Back buttons still arrive as
app-command.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: 4gray <fourgray@proton.me>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/architecture/workspace-shell.md

@@ -97,8 +97,9 @@ The shell is intentionally split into four persistent regions:
        or Sources when the dashboard is off).
 2. Top header:
     1. Leading Back slot: the current page's registered Back, else browser
-       history while an in-app previous page exists, else nothing. See
-       [Header Back](#header-back).
+       history while an in-app previous page exists, else nothing. The mouse
+       Back button, Alt+← / Cmd+[ and the Electron window gestures run it.
+       See [Header Back](#header-back).
     2. Playlist switcher.
     3. Route-aware search input and command palette trigger.
     4. Add source action.
@@ -319,13 +320,80 @@ of jumping (see [Layout slides](#layout-slides)).
 There is no Forward button: Stalker inline details are store state, not
 history entries, so Forward would skip them.
 
+**Platform gestures.** The mouse Back button, Alt+← (Cmd+[ on macOS) and, in
+Electron, the window's `app-command` `browser-backward` (Windows and Linux:
+mouse Back buttons and the Browser Back key) and trackpad `swipe` left (macOS)
+run the same action as the header button, through
+`WorkspaceBackGesturesService` (shell-provided). Each gesture has one of
+three results (`WorkspaceBackGestureResult`):
+
+- `back`: the header has a target and nothing sits above the page, so
+  `WorkspaceBackNavigationService.goBack()` runs. "Above the page" is document
+  fullscreen (a fullscreen player), an open phone context drawer, a showing
+  CDK backdrop, or a visible `[role="dialog"]`, `[role="alertdialog"]` or
+  `[role="menu"]` outside hidden, `inert` or `aria-hidden` subtrees.
+- `blocked`: something sits above the page. Nothing navigates, and the
+  gesture is still swallowed, because browser Back would navigate under the
+  dialog (which then closes) or out of the fullscreen player.
+- `none`: the header has no Back. The gesture stays the browser's: in the PWA
+  it may leave the app, as browser Back does.
+
+Unless the result is `none`, the browser's default is prevented (the mouse
+press on `mouseup` and `auxclick`, listened to in the capture phase), so the
+browser cannot take a second history step. In Electron the mouse Back button
+is prevented even then: Electron's Chromium steps through history on an
+unprevented press itself, also to entries from before a reload, which reloads
+the app.
+
+- **Chord.** `navigate-back` in `keyboard-shortcut-definitions.ts` lists it in
+  the shortcuts dialog; `isWorkspaceBackShortcut()` matches it, on macOS also
+  by the `BracketLeft` key position for layouts that type `[` with Option.
+  `WorkspaceKeyboardShortcutsService` handles it in a capture-phase document
+  listener and stops a chord that runs Back there, so a focused carousel, tab
+  list, guide or player does not also read it as a plain arrow; a blocked
+  chord is prevented but still reaches the dialog or menu. It is ignored
+  while typing (Alt and Option arrows edit text). A held chord runs Back
+  once: its repeats follow its first press until a key is released, so after
+  a press that ran Back they are swallowed even when the page it reached has
+  no Back left, and the browser cannot step on them.
+- **Electron.** `attachWindowBackGestures()`
+  (`apps/electron-backend/src/app/services/window-back-gestures.ts`) forwards
+  the two window events over the main → renderer push channel
+  `WINDOW:BACK_REQUESTED` with the source (`app-command` or `swipe`); the
+  preload's `onWindowBackRequested` passes only those two values on. The
+  renderer subscribes only when
+  `RuntimeCapabilitiesService.supportsWindowBackGestures` reports the
+  subscription, so an older preload forwards nothing. `browser-forward` and
+  `swipe` right are not forwarded. `swipe` needs macOS's "Swipe between
+  pages" set to two or three fingers. On Linux Electron raises `app-command`
+  for the keydown and the keyup of the Browser Back key alike, so the main
+  process takes that key in `before-input-event` instead (Back once per
+  press, the key never reaches the page) and leaves `app-command` to mouse
+  Back buttons.
+- **One press, two events.** In Electron on Windows and Linux a mouse Back
+  press also reaches the window as `app-command`: with the release on
+  Windows, on press on Linux, and before or after the renderer sees the
+  button. Whichever half comes first runs Back and the other is consumed. A
+  command that arrives while the button is down belongs to that press however
+  long it is held; otherwise a command and a press (or a press and a command)
+  within `BACK_PRESS_TWIN_MS` (500 ms) of each other pair. Each press pairs
+  once, so two quick presses still go back twice, and the chord and the swipe
+  never pair.
+
+Electron E2E `back-gestures.e2e.ts` drives each gesture (the mouse through
+CDP's `back` button, the window events emitted on the `BrowserWindow`) and
+checks that each goes back one history entry, a paired press goes back once
+(also when held), Forward does nothing, and the gestures wait while typing
+(the chord) or with a dialog open. Web E2E `workspace-header-back.e2e.ts`
+checks that the mouse Back button and the chord step once in the browser.
+
 **Phone width.** `phoneDrawerToggle` sets how Back shares the leading slot with
 the context drawer toggle. `replace` (the default) takes the toggle's slot: a
 detail page's drawer belongs to the list that Back returns to. `beside` keeps
 both: the settings drawer holds the page's own sections. `yield` hides Back
 while the toggle shows: the history fallback must not cost a category list its
 only way into the drawer, and two navigation icons do not fit beside the
-switcher. System and browser Back still work there.
+switcher. The platform gestures above and browser Back still work there.
 
 **Left in place.** These controls stay inside their surface on purpose:
 
@@ -517,7 +585,8 @@ Keyboard shortcut help is shell-owned:
 
 1. `WorkspaceKeyboardShortcutsService` is provided by `WorkspaceShellComponent`.
    It owns the workspace-scoped `document:keydown` listener for `?` /
-   `Shift+/`.
+   `Shift+/`, and a capture-phase one for the Back chord (see
+   [Header Back](#header-back)).
 2. The listener ignores events from inputs, textareas, selects, and
    content-editable elements via `isTypingInInput(...)`.
 3. `libs/portal/shared/util/src/lib/keyboard-shortcut-definitions.ts` is the